Load transfer switch with non-linear switching resistors



June 12, 1962 B. JANSEN 3,039,041

LOAD TRANSFER SWITCH WITH NON-LINEAR SWITCHING RESISTORS Filed Sept. 24,1957 2 Sheets-Sheet 1 NON LINEAR RES/STOPS A IVO/V 04 54 mas/s70)? 2 /NVEN 7'01 Bernhurd JANSE N United States Patent 1 3,039,041 LOAD TRANSFERSWITCH WITH NON-LINEAR SWITCHING RESISTORS Bernhard Jansen,Prufeningerstr. 20,

Regensburg, Germany Filed Sept. 24, 1957, Ser. No. 685,941 Claimspriority, application Germany Sept. 28, 1956 11 Claims. (Cl. 323-435)The present invention relates to load transfer switches for powerdistribution transformers, the switching device including switchingresistors each of which includes at least one resistor of constantresistance and one nonlinear resistor the resistance of which decreaseswith increasing current flow therethrough.

In most cases transfer switches for changing the taps of a transformeron load consist of a number of pairs of main and auxiliary contacts withinterposed ohmic resistors (so-called switching resistors) which, due tothe alternate closure and opening of the main and auxiliary contactpairs, are briefly connected into the external current circuit of thetap change transformer. During the transfer of the load from onetransformer tap to another the switching resistors are designed toprevent an interruption in the working current and also to prevent ashort-circuit between the transformer taps which may be simultaneouslyactive for a brief period.

The resistors should be so dimensioned that during load transfer theworking current should be fed to the consumer network with the minimumvoltage drop, whilst simultaneously the balancing current prevailingbetween two adjacent taps on the transformer should be limited to atolerable amount. For the first purpose the'switchi'ng resistors shouldhave an ohmic resistance which is as low as possible, and in the secondcase as high as possible, Due to these conflicting requirements acompromise with regard to the magnitude of the resistance value isnecessary in all cases. In addition, a third requirement is that whenconnecting the switching resistors in the path of the systern currentand the balancing current the switching cur-. 4

rents and switching voltages (recovery voltages) prevailing, at the mainand auxiliary contacts employed for this purpose should be as low aspossible, because their magnitudeespecially in the case of extra highvoltage trans-.. formers-to a large extent governs the dimensions of thetransfer switch.

If a certain size of transfer switch and switching resistors has beendecided upon then currents have to be interrupted during the transfer ofload which in some switching positions are governed solely by theprevailing system current, and in other switching positions solely bythe stage voltage between two transformer taps. In intermediateswitching positions the switching currents are governod both by thesystem current and by the stage voltage. The previously mentionedrecovery voltage which apart from the switching current governs arcquenching is, apart from the system current and stage voltage, alsogoverned by the resistance value of the switching resistors.

If the transfer switch is limited solely to the normal operatingconditions of tap changing taps at the transformer operating at normalvoltages and currents, and if special protective gear is installed tocope with abnormal operating conditions such as the occurrence of surgewaves and load transfer during a short-circuit, then in spite of thefact that they have conflicting influences on the required resistancevalue, the initially mentioned functions required of the transfer switchdo lead to economically tolerable designs. If however it is requiredthat a transfer switch should deal with high operating voltages andcurrents, surge waves and short-circuit currents, the principlespreviously employed for the construction of transfer switches result indisproportionately large dimensions and ,high costs.

This invention is intended to provide a better and more economicalmethod of overcoming these difliculties. In accordance with theinvention the ohmic switching resistances employed for changing the tapstransformer during operation are so designed that their resistancevaries with the cur-rent flowing and/or the applied voltage. By thismeans the switching currents and the switching voltages prevailingduring normal transformer operation can be reduced to lower values thanpreviously. Especially when carrying out tap changing under over-currentconditions (in an extreme case during short-circuit) it is possible forthe switching voltages prevailing at the switching contacts to bedecisively reduced. In addition voltage surge waves can be reduced to avalue which is harmless to the transfer switch.

Other objects and advantages will hereinafter appear.

In the drawing forming part of this specification:

FIG. 1 is a diagrammatic view of one form of generator tap transferswitch employing tap resistors which employ variable resistanceelements;

FIG. 2. is similar to but less complete than FIG. 1, and particularlyshows a tap resistor which combines fixed and variable resistanceelements in both series and parallel relationships;

FIGS. 3 and 4 are diagrammatic views which show additional forms of tapresistors which involve both fixed and variable resistance elements inother combinations; and

FIGS. 5 and 6 are diagrammatic views similar to FIG. 1 but showingfurther modified forms of tap resistors in which elements of fixed andvariable resistance are combined.

In FIG. 1 we have shown the general application of variable switchingresistors in conjunction with a transfer switch contact system havingtwo main contacts Ha, Hb and two auxiliary contacts hm, hb, all spacedfrom each other. The contacts are connected with the transformer taps A,B directly via the switching resistors Ra, Rb. The current is tapped offat contact C through a sector contact K. The arrow passing obliquelythrough the switching resistors Ra, Rb indicates that the resistancevaries with current change therethrough. The sector contact K has acurved side whose length exceeds the spacing of each two adjacentcontacts Ha, ha, hb, and Hb. Contact C is located at the apex of contactK and serves as axis of rotation.

Main contact disconnection: In the first switch position shown in FIG.1, in which the center radius of sector contact K extends in thedirection of the dotted arrow at point 1, the current passes from top Athrough the main contact Ha and the sector contact K to the tap C andthen on into the system. The leading radial edge of contact K is justpast point 2. When the load is transferred from tap A to tap B theauxiliary contact ha is first connected up by turning sector contact Kto the left so that at point 3 there is a second path for the currentfrom A through Ra, ha, K to C (second switch position). The transformerloading only proceeds along this second current path when the maincontact Ha which is directly connected with the tap A is opened. Whenthe contact K moves further to the left (counterclockwise in thedrawing) to point 4 an arc is drawn at the main contact Ha, whilst theauxiliary contact he remains closed. As the arc resistance increases atHa an increasing proportion of the In the equation, and c are calculatedconstants, R is resistance, I is current, and U is voltage.

Inthe case exemplified there will, with an 8-fold inc-rease in systemcurrent In, not be an 8-fold increase in recovery voltage as withconstant-value switching resistors, but only a 2-fold increase, because1 is proportional to U. Since the dimensions of transfer switches dependmore on the recovery voltage than on the magnitude of the switchingcurrent, the use of switching resistors which vary in accordance withthe current flowing offers special advantages for this initial circuitbreaking operation at the main contact Ha. The degree of reliability ofthis disconnection at Hot comes into account mainly however withoverload switching (in extreme cases during shortcircuits).

Disconnection of auxiliary contact: when the sector contact K movesfurther from third switch position at point 4 to fourth switch positionat point 5 the second auxiliary contact hb is closed, thus giving riseto a third current path from tap B through Rb, hb, K to C. At thismoment the system current In will be divided between the two taps A andB in a proportion governed by the instantaneous values of the switchingresistors Ra and Rb. However another factor affects the magnitude ofthese two divided currents, because simultaneously with the formation ofthe previously mentioned third current path, there is yeta fourthcurrent path from A through Ra, ha, hb, Rb to B. The difference involtage between A and B (stage voltage Uab') causes a balancing currentI ab to flow along this fourth path which is governed by the sum of theinstantaneous resistance values Ra-l-Rb. This balancing current asdistinct from the previously mentioned divided currents, is independentof the system current In. This balancing current Iab' which is notdesired flows even when tap changing is carried out with the transformeron noload. Where the switching resistors are normally of equal size andconstant i.e. Ra=Rb=R, the magnitude of the balancing current is thesame at both ta-ps A and B. This condition which prevails on no-loadchanges when the transformer is loaded with a system current whichincreases from 0 to In. Due to the equal size of the switching resistorsthe two taps A and B each receive half of In. As a result there is alsoan increase in the divisional current In coming from Ayfrom I ab to Jab+whilst the divisional current I b which comes from B is reduced toreduction side (tap B). As a result the total resistance Ra-l-Rbincreased and this greatly reduces the undesiredbalancing current I ab,which .is governed only by the constant stage voltage Uab on. the onehand and the total of the resistances Ra-l-Rb on the other.Consequently. there will also be a reduction in the switching current Inat the auxiliary contact ha. In addition to this favourable effeet therecovery switching voltage at auxiliary contact ha is reduced whencontact K reaches the fifth switch position at point 6. In thisswitching position the system current In flows completely from tap Bthrough Rb, hb, K to C,

The recovery voltage which consequently prevails between the contactsegment K and the auxiliary contact ha is ,now equal to the sum of thevoltage drop a-t Rb and the stage voltage Ua'b. Whilst the latter isconstant, the voltage drop at Rb is, by employing avariable resistor asdescribed above, severely reduced in the same manner as alreadydescribed for disconnection o-f the main contact. Thus the sameconsiderations and conclusions concerning the suitability of theswitching resistors in accordance with the invention also apply for thedisconnectionof the auxiliary contacts described here. For the sake ofcompleteness only it should be mentioned that the further rotation ofcontact K into sixth position at point 7 and beyond results in thedirect connection of the main contact Hb, and the brief shunting of theswitching resistor Rb. The current now flows from the new tap B throughHb, K to C. By this means the loa-d transfer process is concluded.

In the previous description of FIG. 1 the variable nature of theswitching values of the two switching resistances Ra and Rb wasindicated merely by way of an example, without going into detailconcerning the characteristics of such variability. In a practicaldesign attempts will be made to match the resistance characteristic toconditions prevailing. This can be achieved simply by combiningresistance elements having constant and variable resistance valueswhichcan be selected independently of each other. The various elementscan also be given different variation characteristics and eitherconnected in series or parallel. As an example a constant-value resistorwhich is connected in series with a variablevalue resistor will resultin a minimum resistance limit; 'A parallel-connected constant valueresistor which is connected in parallel with a variable value resistorwill resultin a maximum resistance limit. Similarly theparallel-connection of two variable resistors with diiferentcharacteristic curves will result in an increase in the low resistancevalue and a reduction in the high resistance value. FIG. -2 illustratesa general example for such a combination. For the sake of simplicitythis shows only the upper half of the transfer switch device. The restcan be readily visualized by analogy to FIG. 1, the tap B connectionsforming a mirror image of the tap A connections. Here the variableresistor Rwl is first'connected in parallel with a constant resistorR112. This partial co-mbination first results in a maximum limit on thevariable resistance value. Apart from this however these is also afurther variable value resistor with a different variable resistancecharacteristic, R004 which is connected in series with a constant valueresistor R03 which in turn imposes a minimum limit on'the variableresistance values. Resistors Ra3, Ra4 are additionally connected inparallel as a third unit with the two above-mentioned parallelresistors. With this general arrangement it is possible for allcharacteristic or absolute values of the resistors Ral Ra l to be soarranged that the resistance combination most suitable for theparticular system involved is obtained.

In practice it will usually suffice to employ variable sisters to securemore favourable conditions concerning spare part stocks and the spatiallayout of the switching resistors in the transfer switch. FIGS. 3 and 4show embodiments which permit standard-unit design.

FIG. 3 shows tapping A connections only, the arrangement comprising afixed resistance Ra2 in parallel with a plurality of resistance units,each comprising a fixed resistance Ra3 and a variable resistance Ro4 inseries with one another. This arrangement would be duplicated fortapping B.

FIG. 4 also shows tapping A connections only, the arrangement comprisinga fixed resistance Ra3 in series with a succession of circuit loops,each including a variable resistance Rail in parallel with a fixedresistance Ra2.

All the combinations shown in FIGS. 2-4 of various resistors to form asingle variable switching resistor with a predetermined characteristicenvisage that the various elements are constantly interconnected. Withdifficult cases involving high powers it is however desirable duringload transfer not merely to have two similar, or in some cases combinedswitching resistors, as shown in FIGS. 1 and 2, but to have recourse tomore complex arrangements like those of FIGS. 5 and 6. The descriptionof the stresses during disconnection on the main contact Ho and on theauxiliary contact ha in FIG. 1 has already indicated that in the variouswitching positions diiferent problems have to be overcome concerning thereduction in switching current and the recovery switching voltage. Thisapplies to an even higher degree if transfer switches with more than twodisconnections per load trans-fer operation are to be equipped with suchvariable switching resistors. The combination in accordance with FIG. 5will serve as a simple example. Here in the second, fourth and sixthswitch positions, the resistance combinations Ral-l-RaZ with Ra3+Ra4,Ra3-l-Ra4 with Rb-3 -i-Rb4, Rb3 +Rb4 with Rb1+Rb2 are made during oneload transfer operation one after the other by the auxiliary contacts hal, ha2, 11b2, hbrl at points .1, 3, 5, 7. Due to the interim connectionof three auxiliary contacts ha1+ha2+hb2 at point 5 in third switchposition and ha2+hb2+hb1 at point 7 in fifth switch position, this beingpossible in the transfer switch shown owing to the contact overlap ofwide angle sector contact K, we obtain two further combinations Ral Ra4with Rb3+Rb4 and Ra3+Ra4 with Rb=1 R124. If we compare the resistancecombination Ral Ra4 in FIG. 5 with the resistance combination in 'FIG.2, we see that the only difference lies in the separation of theparallel-connected group Rm1+Ra 2 and the series-connected group Ra3+Ra4by connection at two auxiliary contacts hal and ha2 (FIG. 5) instead ofat only one auxiliary contact ha ('FIG. 2.) Thus by doubling the numberof auxiliary contacts it has become possible, for the same expenditureon resistors, to obtain different parallel and series combinationsduring the load transfer process by means of the intermittentinterconnection of several auxiliary contacts. It is obvious that bythis means it is possible to adapt the characteristics and the absolutevalues of the resistance combinations to the appropriate switchingpurposes at the individual main or auxiliary contact.

In the arrangement shown in FIG. 5 these different combinations wereobtained by the positive sequence of main and auxiliary contactinterconnections with the aid of the sector contact K. However in loadtransfer switch practice there are also cases in which it is desirableto make such resistance combinations only in special operating cases,such as overcurrents due to shortcircuit and overvoltage due tolightning stroke, which do not occur in normal operation. This can beobtained by installing additional automatic-acting switching elements.In FIG. 6 we have shown one embodiment in which, in the event of a surgevoltage wave, the spark gaps fab, fw, fb are actuated and cause theparallel or series connection of the switching resistors Ral or Rbl withthe constant value resistors RaZ or Rb2. When the surge has died awaythe 6 spark gaps are again quenched and the normal load transfer processmakes use in the normal way solely of the switching resistors Ra2 andRb2. These spar-k gaps can also respond in the event of an overcurrent,because the overcurrent produces a voltage drop in the constant valueresistors Ra l or Rb2 which supplies the corresponding striking voltageat the spark gaps. As a result the variable resistors Ral or Rbl areconnected in parallel and this results in the extremely marked reductionin the resistance of the combination, already discussed in connectionwith FIG. 1, which also reduces the recovery switching voltage atcontacts Ha or ha to a value which can more easily be dealth with by thetransfer switch.

What I claim is:

l. A switching circuit for selective connecting one of two taps of apower supply device to an external circuit, comprising a plurality offixed contacts disposed in a spaced arcuate array, said array includingtwo main end contacts and a plurality of auxiliary contacts disposedbetween the two end contacts, one of the auxiliary contacts beingconnected via a purely ohmic resistive variable resistance circuit toone of the main end contacts, another of the auxiliary contacts beingconnected via another purely ohmic resistive variable resistance circuitto the other main end contact, each of said variable resistance circuitsincluding a resistor of constant ohmic resistance, and a none-resistiverotatable contact having a curved end contact portion movable in contactwith said fixed contacts, said contact portion being longer than thespacing between each two adjacent fixed contacts so that said contactportion contacts at least two of said fixed contacts in all switchingpositions thereof between the end contacts, there being additionalpurely ohmic resistive variable resistance circuits respectivelyconnected between further ones of said auxiliary contacts and said endcontacts, said additional circuits being respectively connected inparallel to respective ones of the first named variable resistancecircuits when said contact portion simultaneously contacts one of thefirs-t named auxiliary contacts and one of said further auxiliarycontacts.

2. A switching circuit according to claim 1 wherein each of saidvariable resistance circuits comprises a resistor the resistance ofwhich decreases with increasing current flow therethrough.

3. A switching circuit according to claim 2, further comprising sparkgap means connecting each resistor of constant ohmic resistance into thevariable resistance circuit in which it is included.

4. A switching device for changing transformer taps under load,comprising first and second main contacts connected, respectively, totwo transformer taps, a movable switching member separately engageablewith either of said main contacts, first and second intermediatecontacts successively engageable by said switching member during itsmovement from engagement with said first main contact into engagementwith said second main contact, said switching member, during saidmovement, establishing contact with said first main contact, said firstintermediate contact, said second intermediate contact and said secondmain contact with make before-break sequence in the order named and inthe inverse order during return movement, a firs-t resistance meansconnected between said first main contact and said'first intermediatecontact and a second resistance means connected between said second maincontact and said second intermediate contact, both of said resistancemeans comprising a first resistor the resistance of which decreases withincreasing current flow therethrough and a second resistor of constantresistance.

5. A device according to claim 4 wherein in each resistance means, saidresistors are connected in series.

6. A device according to claim 4, wherein in each resistance means, saidresistors are connected in parallel.

7. A device according to claim 4, comprising, within each resistancemeans, a second resistor and at least one 7 further first resistor andone further second resistor, said further resistors being connected inparallel.

8. A device according to claim 4, comprising, within each resistancemeans, asecond resistor, at least one further first resistor and atleast one further second resistor, said further resistors beingconnected in series.

9.- A device according to claim 4, further comprising spark gap meansinterconnecting said first and second resistors.

10. A device for changing transformer taps under load, comprising, afirst main contact connected to a first transformer tap, a second maincontact connected to a second trans-former tap, a movable switchingmember selectively engageable only separately with either of said maincontacts, at least four intermediate contacts arranged for sequentialengagement by said switching member during its movement from engagementwith one of said main contacts into engagement with'the other maincontact, afirst resistance means connected from a first intermediatecontact to said first main contact, a second resistance means connectedbetween a second one of said intermediate contacts and said first maincontact, a third resistance means connected between a third one of saidintermediate contacts and said second main contact, and a fourthresistance meansconnected between a fourth one of said intermediatecontacts and said second main contact, said switching member duringmovement between engagement with said first main contact and said secondmain contact engaging successively said first, second, third and fourthintermediate cont-acts in the order named with make-beforebreak sequenceand in the reverse order during return movement, each of said resistancemeans including a resistor of constant resistance and a resistor theresistance of which decreases with increasing current flow thcrethrough.

11. A device according to claim 10, wherein said switching member at alltimes simultaneously engages at least two of said contacts.

References Cited in the file of this patent UNITED STATES PATENTS745,379 Pearson etal. Dec. 1, 190 3 20 2,063,693 McCarty Dec. 8, 19362,138,652 Biermanns Nov. 29, 1938 2,276,855 Meador Mar. 17, 19422,435,438' Fowler Feb. 3, 1948 2,680,790 Jansen June 8, 1954

